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Date: Tue, 10 Aug 2021 18:30:21 +0800
From: Lai Jiangshan <laijs@...ux.alibaba.com>
To: Paolo Bonzini <pbonzini@...hat.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
linux-kernel@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
kvm@...r.kernel.org
Subject: Re: [PATCH V2 2/3] KVM: X86: Set the hardware DR6 only when
KVM_DEBUGREG_WONT_EXIT
On 2021/8/10 18:07, Paolo Bonzini wrote:
> On 09/08/21 19:43, Lai Jiangshan wrote:
>> From: Lai Jiangshan <laijs@...ux.alibaba.com>
>>
>> Commit c77fb5fe6f03 ("KVM: x86: Allow the guest to run with dirty debug
>> registers") allows the guest accessing to DRs without exiting when
>> KVM_DEBUGREG_WONT_EXIT and we need to ensure that they are synchronized
>> on entry to the guest---including DR6 that was not synced before the commit.
>>
>> But the commit sets the hardware DR6 not only when KVM_DEBUGREG_WONT_EXIT,
>> but also when KVM_DEBUGREG_BP_ENABLED. The second case is unnecessary
>> and just leads to a more case which leaks stale DR6 to the host which has
>> to be resolved by unconditionally reseting DR6 in kvm_arch_vcpu_put().
>>
>> We'd better to set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT,
>> so that we can fine-grain control the cases when we need to reset it
>> which is done in later patch.
>>
>> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
>> ---
>> arch/x86/kvm/x86.c | 4 +++-
>> 1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index ad47a09ce307..d2aa49722064 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -9598,7 +9598,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>> set_debugreg(vcpu->arch.eff_db[1], 1);
>> set_debugreg(vcpu->arch.eff_db[2], 2);
>> set_debugreg(vcpu->arch.eff_db[3], 3);
>> - set_debugreg(vcpu->arch.dr6, 6);
>> + /* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */
>> + if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)
>> + set_debugreg(vcpu->arch.dr6, 6);
>> } else if (unlikely(hw_breakpoint_active())) {
>> set_debugreg(0, 7);
>> }
>>
>
> Even better, this should be moved to vmx.c's vcpu_enter_guest. This
> matches the handling in svm.c:
>
> /*
> * Run with all-zero DR6 unless needed, so that we can get the exact cause
> * of a #DB.
> */
> if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT))
> svm_set_dr6(svm, vcpu->arch.dr6);
> else
> svm_set_dr6(svm, DR6_ACTIVE_LOW);
>
> That is,
>
> KVM: X86: Set the hardware DR6 only when KVM_DEBUGREG_WONT_EXIT
> Commit c77fb5fe6f03 ("KVM: x86: Allow the guest to run with dirty debug
> registers") allows the guest accessing to DRs without exiting when
> KVM_DEBUGREG_WONT_EXIT and we need to ensure that they are synchronized
> on entry to the guest---including DR6 that was not synced before the commit.
> But the commit sets the hardware DR6 not only when KVM_DEBUGREG_WONT_EXIT,
> but also when KVM_DEBUGREG_BP_ENABLED. The second case is unnecessary
> and just leads to a more case which leaks stale DR6 to the host which has
> to be resolved by unconditionally reseting DR6 in kvm_arch_vcpu_put().
> Even if KVM_DEBUGREG_WONT_EXIT, however, setting the host DR6 only matters
> on VMX because SVM always uses the DR6 value from the VMCB. So move this
> line to vmx.c and make it conditional on KVM_DEBUGREG_WONT_EXIT.
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index ae8e62df16dd..21a3ef3012cf 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6625,6 +6625,10 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
> vmx->loaded_vmcs->host_state.cr4 = cr4;
> }
>
> + /* When KVM_DEBUGREG_WONT_EXIT, dr6 is accessible in guest. */
> + if (vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)
> + set_debugreg(vcpu->arch.dr6, 6);
I also noticed the related code in svm.c, but I refrained myself
to add a new branch in vmx_vcpu_run(). But after I see you put
the code of resetting dr6 in vmx_sync_dirty_debug_regs(), the whole
solution is much clean and better.
And if any chance you are also concern about the additional branch,
could you add a new callback to set dr6 and call the callback from
x86.c when KVM_DEBUGREG_WONT_EXIT.
The possible implementation of the callback:
for vmx: set_debugreg(vcpu->arch.dr6, 6);
for svm: svm_set_dr6(svm, vcpu->arch.dr6);
and always do svm_set_dr6(svm, DR6_ACTIVE_LOW); at the end of the
svm_handle_exit().
Thanks
Lai
> +
> /* When single-stepping over STI and MOV SS, we must clear the
> * corresponding interruptibility bits in the guest state. Otherwise
> * vmentry fails as it then expects bit 14 (BS) in pending debug
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a111899ab2b4..fbc536b21585 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9597,7 +9597,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> set_debugreg(vcpu->arch.eff_db[1], 1);
> set_debugreg(vcpu->arch.eff_db[2], 2);
> set_debugreg(vcpu->arch.eff_db[3], 3);
> - set_debugreg(vcpu->arch.dr6, 6);
> } else if (unlikely(hw_breakpoint_active())) {
> set_debugreg(0, 7);
> }
>
> Paolo
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